load_current DOUBLE The amount of work that the database server is currently performing.
load_last_1_min DOUBLE The amount of work that the database server has performed in the previous minute.
load_last_5_mins DOUBLE The amount of work that the database server has performed in the previous 5 minutes.
load_last_10_mins DOUBLE The amount of work that the database server has performed in the previous 10 minutes.

each server fires an internal event each second to capture and record the average load on the server during the past second (the load average is computed by the database server kernel - it basically sums up the number of active requests divided by the number of concurrent threads (i.e. cores) being used)

every few seconds the stats from each mirror server is sent to the server's parent (in the 'tree' of copy/mirror nodes); an intermediate node passes the data onto its parent, and the primary compiles the stats of all servers.

a call to sa_mirror_server_status simply returns the information that has been compiled.

If a server is not involved in any mirrored databases then calling sa_mirror_server_status will not return any useful information since there is no information being computed and accumulated.